Parents beware: Giving in to teens’ demands for their own cars can have dangerous consequences, new research suggests.

Teenagers with their own cars or free use of one are much more likely to get in crashes than those who share a car. And crashes are much less common among teens whose parents set clear driving safety rules.

The findings are in two studies by researchers at Children’s Hospital of Philadelphia and funded by State Farm Insurance Co. They were released Friday and are in the October issue of Pediatrics.

The researchers say the findings can help parents keep their kids from becoming a grim statistic: Traffic crashes are the leading cause of death for U.S. teens, killing more than 5,000 each year.

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Getting a driver’s license and car are often viewed as rite of passage for U.S. teens, and many parents underestimate the risks.

More than 7,000 people nationwide were killed in crashes involving teen drivers in 2007, government data show. More than 3,000 of these deaths were teen drivers, and more than 250,000 teen drivers were injured.

“With teen drivers, you have to recognize that it’s a public health issue,” said Dr. Jeffrey Weiss, a Phoenix pediatrician who co-wrote an American Academy of Pediatrics report on teen drivers.

The 2006 report encourages parents to highlight the seriousness of driving privileges by requiring teens to sign driving contracts promising to abide by safety rules.

The new research shows that kind of hands-on approach pays off.

“Families need to know that driving is different” from other steps toward independence,said Dr. Flaura Koplin Winston, the study’s lead author. “Just at the time their teen is pulling away, they need to get back involved to spare them heartache.”

The research is based on a nationally representative survey of more than 5,500 teens in grades nine through 11. Students at 68 high schools answered questionnaires in 2006.

More than 2,000 students who reported driving on their own were the focus of one study; 70 percent said they had their own cars or were the main drivers of cars they used.

Winston said it’s alarming that so many kids have their own cars or feel that they have free use of one. She said that freedom can lead to “a sense of entitlement about driving” that may make them less cautious.

Among these “main” drivers, 25 percent had been involved in crashes, versus just 10 percent of teens who shared driving access. Winston said the lower crash rate doesn’t reflect less driving time, but is likely due to having to ask for the car keys, which helps parents monitor their kids’ driving.

Compared with teens whose parents were uninvolved, kids who said their parents set clear rules and monitored their whereabouts without being overly controlling had half as many crashes and much better driving habits.

These teens were 71 percent less likely to drive while drunk and 30 percent less likely to use a cell phone while driving than kids with uninvolved parents.

Dr. Niranjan Karnik, a University of Chicago specialist in adolescent mental health, said the research underscores the importance of appropriate parenting and widely enacted graduated licensing laws for teens.

Debby Hendricks of Hatfield, Pa. made her daughters wait until age 17 to get their licenses, and gave them lots of driving practice beforehand.

The girls, aged 17 and 19, also share a family car, and can’t “just grab the keys and leave” without saying where they’re going and with whom, Hendricks said.

So far so good – neither girl has been in an accident, although the younger one, Leslie, has only had her license for a few months.